How to scale a side of already subdivided prism?
There are various tool that can help you in this kind of operation. Here's four that come in my mind:


Proportional edit:



  Proportional Edit is a way of transforming selected elements (such as
  vertices) while having that transformation affect other nearby
  elements


While in edit mode, enable Proportional editing with the faces to scale selected. Choose "linear" interpolation type, scale on the wanted axis and than adjust the influence radius with the mousewheel.




Lattice modifier



  A lattice consists of a three-dimensional non-renderable grid of vertices. Its main use is to give extra deformation capabilities to the underlying object it controls 


While in object mode, add a Lattice object to the scene and set its' vetex number to 4. Position the lattice upon the starting mesh and scale it as it would be the obect's bounding box. Add a Lattice modifier pointing to your lattice to the objecte. Then enter in Lattice's edit mode and move the top vertices toward the center.




Simple deform modifier - Taper



  Linearly scales along Z axis.


Add the modifier to your obhect and choose Taper mode. Lock X scale (or Y) to prevent deformation on both axes and adjust other parameters on your need.




Shear (or scale on axis) the whole face